This invention relates to a multi-grip blind rivet, and particularly relates to a multi-grip blind rivet having one or more radial indentations formed in one or more transaxial planes of a rivet body of the rivet.
Typically, multi-grip blind rivets include a mandrel, or stem, formed with a head at one axial end thereof, and a rivet body, formed as a sleeve or shank having a passage therethrough and a flange at one axial end thereof. A mandrel-body assembly is formed by inserting a non-head end of the mandrel into the passage at a non-flange end of the rivet body and through the passage. With this assembly, the head of the mandrel is eventually resting against the non-flange end of the rivet body and the non-head end of the mandrel is extending outward from the flange end of the body. The assembly is then processed through a forming operation where a groove is, or plural indentations are, formed in the outer periphery of the rivet body in a given transaxial plane. A plurality of grooves could be formed in the outer periphery of the rivet body, in spaced transaxial planes, along the axial length of the body in lieu of the indentations.
In known multi-grip rivets of the type described above, the grooves are continuous, and concave with respect to outer surface of the shank, with a concave work hardened surface formed at the base of the groove. With respect to the known multi-grip rivets which are formed with indentations, each such indentation, and the base thereof, could be concave with a concave work hardened base. Or, the indentations could be formed with flared upper and lower walls extending from a flat work hardened surface, which is in the form of a secant to the circumference of the shank of the rivet body.
Subsequent to the formation of the grooves in the rivet body, the mandrel-body assembly is placed within aligned openings of two or more workpieces to be secured together by the rivet. The diameter of the aligned openings is greater than the diameter of the external surface of the shank of the rivet body. In this position, an outer surface of one of the workpieces is in interfacing engagement with an underside surface of the flange of the rivet body. Thereafter, while supporting the flange of the rivet by the nosepiece of the setting tool, the setting of the rivet begins by applying a pulling load, in an axial direction away from the flange, on the portion of the mandrel which extends outward from the shank of the rivet body. The pulling load on the mandrel is transferred to the rivet body shank under a compressive load.
Continued application of the pulling load results initially in the outward radial bulging of the rivet body located axially between the spaced planar grooves followed by near axial closing of each of the grooves and a corresponding axial shortening of the rivet body. As the outwardly bulging section of the rivet body contacts the outer surface of the workpiece, and with the shortening of the axial length of the rivet body, the workpiece parts are clamped together.
Eventually, the multi-grip blind rivet has been set whereby the bulging sections of the rivet body are in firm, distorting engagement with the inner walls of the aligned holes of the workpieces, which creates a gripping action between the bulging sections and the inner walls to retain the workpieces together.
The setting performance of the above-described multi-grip blind rivet, commonly configured either with grooves or swaged indentations along the rivet body, does not perform well where the materials of the workpieces are soft and/or friable, especially where the holes are oversize and there are extremes of grip thickness.
A common form of multi-grip blind rivet, as illustrated and described in U.S. Pat. Nos. 4,958,971 and 6,004,086, has circumferential grooves, usually hemispherical in shape, spaced at intervals along the axial length of the shank of the rivet body. The grooves are concave with respect to the outer surface of the shank of the rivet body. As this type of blind rivet is being set, the plain cylindrical rivet body portions between the grooves expand radially to form a characteristic “cottage loaf” setting and the grooves collapse in an axial direction giving the capability for wider workpiece thicknesses.
As further illustrated and described in U.S. Pat. No. 6,254,324, one or more reinforcing ribs may be formed within the circumferential concave groove, or between circumferential-groove-like indentations, and extend radially from the base of the groove to the outer periphery of the shank of the rivet body.
The control of the depth of the grooves is critical to accommodate the extreme conditions mentioned. For example, if the groove is too deep, cracking of the base of the groove will occur upon setting of the rivet. If the groove is too shallow, there will not be sufficient resistance to prevent the mandrel head from pulling into, or even through, the rivet body.
With respect to the above-noted U.S. patents, a work hardened area may be formed at the base, or bottom, of the groove and will contribute to the development of a resistance to the pulling of the mandrel head into the rivet body. Also, the load being exerted by the mandrel head is transposed to the rivet body and to the groove profile. This load increases under the action of the setting tool causing the rivet body to expand radially between the grooves as the body shortens. The setting process reaches a point where groove closes or collapse, thereby indicating the completion of the setting of the rivet.
Small differences in the groove depth of the grooves, as illustrated and described in the above-noted U.S. patents, can give significant differences in the collapse load. This condition, together with normal manufacturing variations in mandrel crimp breakload, could lead to either premature rivet body failure or the pulling of the mandrel head into the rivet body.
In another type of multi-grip rivet, as illustrated and described in U.S. Pat. No. 5,496,140, indentations are formed at predetermined distances along the shank of the rivet body, with flared upper and lower walls extending outward from a flat work hardened base, which is in the form of a secant to the circumference of the shank. During the setting operation of this type of rivet, the indented portions retain their integrity and allow the intermediate portions between the indentations to collapse.
With respect to the rivet illustrated and described in U.S. Pat. No. 5,496,140, after the mandrel-body assembly has been formed as described above, the rivet body is indented with a series of flat-bottom indentations, such as four equispaced identical indentations in a common transaxial plane around the rivet body. The formation of the indentations provide four thickened portions with work hardened zones therebetween. This structure avoids fully circumferential work hardening and the subsequent risk of rivet body cracking during the application of the mandrel setting load.
As the rivet-setting load is being applied in the setting of the rivet of U.S. Pat. No. 5,495,140, the rivet section, which includes the four thickened portions and the indentations, is supporting the setting load. Again, there is a point in the setting profile where the setting load overcomes the resistance of the work hardened zones and the indentations collapse to complete the setting of the rivet. With this type of rivet, body rupturing is avoided, but only small variations in indentation depth and mandrel crimp break load could result in an incomplete setting or the mandrel head being pulled through the rivet body.
When setting rivets of the types described above, there are two basic concerns which must be addressed:                a. During setting in softer workpiece materials, the grooves or indentations are required to be deeper than normal to prevent the mandrel head from pulling through the rivet body. However, this structure can cause fracture of the rivet body, due partly to the thinness of the material of the body and partly to the degree of material work hardening. If the grooves or indentations are not at an optimum depth, the mandrel can be pulled into the rivet body; and        b. If the grooves or indentations are less than optimum during the setting operation, the mandrel head can be pulled through the rivet body due to insufficient resistance being provided by the rivet body, and little or no resistance being provided by the soft workpiece materials. In extreme cases, the result is that the portion of the mandrel shaft extending from the mandrel head can protrude beyond the flange, thereby creating a potentially hazardous condition. Such a condition could cause excessive radial expansion of the rivet body resulting in splitting of the workpiece material as the radial expansion takes place within the hole of the workpiece.        